Any type of physical anger is wrong. I truly believe in taking a moment and calming down before things are said/done. HOWEVER, I have slapped and ex for cheating. I was wrong. But i'm not perfect and neither is your girlfriend. That is actually the reason why I have tried to be more calm during fights. What you did was wrong. I am not here to shame you, but please listen to my advice. I was cheated on 3 times by my ex, we have not seen each other for 7 years, and I am happily married now. But I still remember the hurt. Understand that your girlfriend is going through a lot right now. When I was cheated on I felt ugly... I felt disappointed, I questioned myself, I worried, and I did not trust my partner anymore. I stayed with him, and let his cheating continue. But the last and final time, I came up with a new plan: don't think about it. Don't ask about 'her', don't look at her pictures, don't wonder where he is, don't ask why you ant good enough and he had to resort for someone else. Your girlfriend is going through a lot right now, and sympathize for her. Chances are, she will be like I was: always worrying, and obsessing about this girl/why this happened. If you are on good terms, PLEASE tell her this. The cheating hurt me, but what broke me was my own mind looking for answers. I needed to know why she was so much better, I needed to know if she meant nothing, and it broke me to obsess this way; its not healthy at all!! If you can, show this to your girlfriend, or find some website, some way to help her heal properly... what I did only hurt me more, and I pray she won't do the same (sadly, many women do). I just had to give you this advice, so you understand what pain she is going through, and hopefully it will help your relationship.
Going back to her slapping you. No one is perfect. To this day, I still believe my ex deserved that slap, BUT I do realize that I could have gotten my point across without it. Your girlfriend is pissed to the EXTREMEEEE. You don't know how many tears she has cried on her pillow thinking of you kissing this other girl. Even when she tells you she feels better, she is spending her lonely nights hugging her pillow tight, with a knot in her tummy, crying out for help. Give her time!!! The last thing she needs right now is worrying about apologizing for her slap. This will only make her more irritated and more fights will come from this. She will say sorry eventually. She loves you.. give her time to heal. I hope you two make it.